Inductive power transmission has been used extensively over the past decades for contactless energy transfer to power a device or recharge its battery. It has covered a wide range of applications with different power requirements from μW to kW. Some examples include powering radio frequency identification (RFID) tags and implantable medical devices (IMDs), and recharging batteries of handheld mobile devices and electric vehicles. FIG. 1 shows a generic model of a conventional inductive power transmission link. It includes a rectifier or a voltage multiplier depending on the voltage amplitude across L2C2-tank, i.e., VR. The power management also includes a regulator (not shown). In an inductive power transmission link, as shown in FIG. 1, an efficient power amplifier (PA) drives the transmitter (Tx) coil, which is mutually coupled to a receiver (Rx) coil. A power management is required to rectify and regulate the AC voltage across the L2C2-tank (VR) to a constant DC voltage across the load (RL), i.e., VL in FIG. 1.
In general, there are four key parameters in inductive power transmission: (1) power delivered to the load (PDL), defined as PL=VL2/RL; (2) power transmission efficiency (PTE), defined as PL/PS, where PS is the PA output power; (3) power conversion efficiency (PCE) within Rx, defined as PL/PR, where PR is the power management input power; and (4) voltage conversion efficiency (VCE) in Rx, defined as VL/VR,peak, where VR,peak is the amplitude of VR.
While achieving high PTE and sufficient PDL should always be considered in the design of inductive links, maximizing PCE or VCE depends on VR. When VR is larger than the required VL, which is the case when coupling distance (d) is relatively small and coils are well aligned, high PCE is more desirable to maximize the power efficiency within Rx, and VCE<1 V/V is quite acceptable. However, for VR<VL with large d and/or misaligned coils, VCE>1 V/V is paramount to achieve the required VL even at the cost of lower PCE. Therefore, for most wireless power transmission (WPT) applications that involve d and coil orientation (ϕ) variations, the power management should be able to sense VR and decide to whether maximize PCE or VCE.
The mutual coupling between a pair of coupled coils, k12, is inversely proportional to d3. A key requirement in all of the aforementioned applications is to provide sufficient VL, while maintaining high PTE. In worst-case conditions when d is relatively large, then the coils are misaligned, or the Rx coil is miniaturized. It should also be noted that even in some low-power applications such as neural stimulators, a relatively high VL is often required. In these conditions, one can increase the PA voltage, Vs, to further increase VL. In practice, however, VL can only be increased to the extent that the tissue exposure to the electromagnetic field is maintained within safety limits, and regulatory requirements for interference with nearby electronics are satisfied. Therefore, achieving sufficient VL at large distances is quite challenging.
The PTE of the 2-coil link in FIG. 1 is also highly sensitive to RL, which is often given by the application. In order to improve PTE for any RL, multi-coil links in the form of 3- and 4-coil links have been proposed that provide load matching inside Rx. However, these links need an additional coil in the Rx, which adds to the size, cost, and complexity of the system. In some applications, RL can change significantly during the operation while 3- and 4-coil links cannot dynamically compensate for RL variations during the system operation. Alternatively, off-chip matching circuits can also be used to transform RL. However, a network of off-chip capacitors and inductors is needed to dynamically tune a wide range of RL during the operation, which again adds to the size, cost, and power loss in the Rx. Therefore, the power management should also provide optimal load condition during the operation.
In order to improve the PCE within Rx, active rectifiers with high-speed synchronous comparators, some equipped with delay compensation, have been presented in recent years. A 3-level reconfigurable resonant regulating rectifier simultaneously rectifies and regulates VL by switching between full-bridge, half-bridge, and no rectifier structures. A resonant regulation rectifier may employ pulse-width/frequency modulation to adjust the on-time window of the active rectifier switch for self-regulating VL, by controlling the forward current. Although high PCE and self-regulation have been achieved in active rectifiers, they suffer from low VCE<1 V/V due to the voltage drop across the active switch.
In order to improve VCE, voltage doublers, multipliers, and DC-DC converters have been presented in the past. The power-management structure may also be switched between rectifier and doubler for voltage regulation and range extension. Although these techniques can improve VCE, they require additional AC-DC converters and/or off-chip components due to the low-frequency operation of the inductive links (<20 MHz), adding to the size, cost, and power loss in the Rx. A common theme with the aforementioned power managements is that they use the Rx LC-tank as a voltage source, i.e., they operate in voltage mode (VM), inherently leading to limited VCE.